The Web server with mimic construction is a new Web security defense system based on the principle of mimic defence. It uses the heterogeneity, dynamics, redundancy, and other characteristics to block or disrupt network attacks to control the security risk of the system. This study analyzes how heterogeneity can improve the security of the Web server with mimic construction and points out the importance of quantification of heterogeneity. Based on the quantification methods of biodiversity, this study defines the heterogeneity of the Web servers with mimic construction as the complexity and disparity of its execution set, proposes a quantification method that is suitable for quantitative heterogeneity, and analyzes the factors that influence heterogeneity of the Web servers with mimic construction. This study provides a new method for quantitative assessment of mimic defence in theory, and provides guidance for choosing the redundancy, components, and execution in practice. The experimental results show that the proposed method is more suitable for quantifying the heterogeneity of Web server with mimic construction than the Shannon-Wiener index and Simpson index.